Characterization of zinc oxide single crystals for epitaxial wafer applications
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Characterization of zinc oxide single crystals for epitaxial wafer applications Naoki Ohashi,1 Takeshi Ohgaki,1 Shigeaki Sugimura,1,2 Katsumi Maeda,2 Isao Sakaguchi,1 Haruki Ryoken,1,3 Ikuo Niikura,2 Mitsuru Sato,2 and Hajime Haneda1 1
National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan 2 Tokyo Denpa Co., Ltd. (TEW), 5-6-11, Chuo Ota-ku, Tokyo 143-0024, Japan 3 Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-0022 Japan ABSTRACT Zinc oxide (ZnO) single crystals were grown by the hydrothermal method using lithium and potassium hydroxide as mineralizer and properties of the grown crystals were characterized from the viewpoints of epitaxial wafer applications. The growth sector dependence of impurity and defect concentrations were characterized by secondary ion mass spectroscopy and photoluminescence. As a result, it was clearly shown that defect and impurity distribution in the obtained crystal was anisotropic, and this anisotropy is affected by the choice of the seed crystal shape and growth direction. Annealing effect on flatness of the wafer surface was also examined, and it was found that high temperature annealing with flat single crystalline cover is appropriate for removal of scratch and formation of atomically flat surface. Moreover, we show the possible miss-evaluation of Hall coefficient of ZnO due to anisotropy in defects and impurities distributions. INTRODUCTION Zinc oxide is one of the candidate materials for wide-band-gap electronics, such as transparent field effect transistors.[1,2] In order to obtain high quality of ZnO based devices, homo-epitaxy using high quality ZnO substrate is the most appropriate way. There are several prior studies on growth of ZnO single crystals, seeded chemical vapor transport (SCVT)[3] method, flux method,[4] and hydrothermal method.[5] In this study, we examined hydrothermal method as a way to obtain large bulk crystal. It is well known that ZnO single crystal grown by hydrothermal method contains lithium (Li) impurity. Indeed, Li is dismissible element for hydrothermal growth of ZnO. As reported in the prior studies, for example in ref. 5, the properties of ZnO grown by the hydrothermal method have anisotropy in their impurity and defect concentration. In order to improve quality of the crystals, shape of the seed crystals and the controlling of the growth direction are of great importance. Thus, we examined the growth direction and seed shape dependence of the impurity concentration and defect concentration. From the viewpoint of substrates for thin film growth, atomically flat surface is also an important issue. In order to realize atomically flat surface without damage layer, we examined thermal annealing at high temperature. It is well-known that vapor pressure of Zn is very high, so that ZnO can be easily evaporated at the higher temperature. It is why SCVT method is appropriate to obtain ZnO single crystals. Taking this situation into account, we investigated the effect of covers on ZnO single crystals
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